This invention relates to digital electronic signaling systems. More particularly, this invention relates to digital systems for generating dual frequency tone signals in time division multiplexed telephone systems.
With the advent of pulse code modulated time division multiplexed (PCM/TDM) telephone systems, such as the EPABX manufactured and sold by the ROLM Corporation of Cupertino, California the need has arisen for an all digital dual frequency tone generator for generating dual tone multifrequency (DTMF) signals in all digital form representative of the digits 0-9, internal and external dial tone, busy signal, and other arbitrary DTMF command signals, e.g., *, #, etc. In the past, attempts have been made to generate such all digital signals by first generating the analog equivalent signal and then digitizing the analog signals in conventional fashion. This technique suffers from the disadvantage of being relatively expensive to implement and relatively unreliable in performance due in part to the opportunity for injection of switching noise into the signals. Accordingly, recent efforts have been directed toward devising an all digital system for generating DTMF signals.
U.S. Pat. No. 3,706,855 illustrates one solution to the problem of generating all digital DTMF signals. However, this solution is limited to those dual frequency signals in which the phase difference between the two basic frequencies of a tone pair is always constant, i.e., to those pairs of tone frequencies which are harmonically related so that mixed pairs of signals are both periodic and symmetric. Although the all-digital DTMF tone generator described in this patent does provide useful results and eliminates the necessity for much analog equipment, the requirement that the DTMF signals be harmonically related in the manner noted above unduly limits the application of the system.